Abstract
Abstract. We describe the effects of nitrogen oxide (NOx) and organic reactivity reductions on the frequency of high ozone days in California's San Joaquin Valley. We use sixteen years of observations of ozone, nitrogen oxides, and temperature at sites upwind, within, and downwind of three cities to assess the probability of exceeding the California 8-h average ozone standard of 70.4 ppb at each location. The comprehensive data records in the region and the steep decreases in emissions over the last decade are sufficient to constrain the relative import of NOx and organic reactivity reductions on the frequency of violations. We show that high ozone has a large component that is due to local production, as the probability of exceeding the state standard is lowest for each city at the upwind site, increases in the city center, is highest at downwind locations, and then decreases at the receptor city to the south. We see that reductions in organic reactivity have been very effective in the central and northern regions of the San Joaquin but less so in the southern portion of the Valley. We find evidence for two distinct categories of reactivity sources: one source that has decreased and dominates at moderate temperatures, and a second source that dominates at high temperatures, particularly in the southern San Joaquin, and has not changed over the last twelve years. We show that NOx reductions are already effective or are poised to become so in the southern and central Valley, where violations are most frequent, as conditions in these regions have or are transitioning to NOx-limited chemistry when temperatures are hottest and high ozone most probable.
Highlights
Ozone formation is a nonlinear function of nitrogen oxides (NOx) and the reactivity of gas phase organic molecules and reductions in the emissions of these precursors can decrease, increase, or leave unchanged the rate of ozone production
We describe ozone’s dependence on NOx and organic reactivity (VOCR) in San Joaquin Valley California using sixteen years of routine measurements of O3, NO2*, and temperature
We show that local ozone production plays a large role in the frequency of high ozone days, as the exceedance percentage is seen to increase from upwind to downwind within each of our study regions and because the probability of a violation between regions is, in each case, higher at the downwind site to the north than at the receptor city to the south
Summary
Ozone formation is a nonlinear function of nitrogen oxides (NOx) and the reactivity of gas phase organic molecules and reductions in the emissions of these precursors can decrease, increase, or leave unchanged the rate of ozone production. At many locations there are reports of decreases in organic emissions (e.g. Environmental Protection Agency, 2003; Parrish, 2006; Bishop and Stedman, 2008; Monks et al, 2009; Wilson et al, 2012) but changes to the total organic reactivity are not well documented These precursor changes are predicted to have substantially affected the photochemical ozone production rate and the probability of exceeding health-based standards. Thornton, et al, 2002; Martinez et al, 2003; Ren et al, 2003), and, very recently, the direct measurement of the instantaneous ozone formation rate (Cazorla and Brune, 2010; Cazorla et al, 2012) These methods each work to constrain the chemistry of ozone production at the specific local NOx and organic reactivity.
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